One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations over the disc, and electrical circuitry that is used to write and read data to and from the disc. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disc.
Information representative of data is stored on the surface of the memory disc. Disc drive systems read and write information stored on tracks on memory discs. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the memory disc, read and write information on the memory discs when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disc. The transducer is also said to be moved to a target track. As the memory disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disc. Similarly, reading data on a memory disc is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disc. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of a disc drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The transducer is typically housed within the slider. The slider is a small ceramic block which is passed over the disc in a transducing relationship with the disc. The small ceramic block, also referred to as a slider, is usually aerodynamically designed so that it flies over the disc. Most sliders have an airbearing surface ("ABS") which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disc. At the same time, the air rushing past the depression in the air-bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular fly height. The fly height is the thickness of the air lubrication film or the distance between the disc surface and the transducing head. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disc.
When the disc is operating, the disc is usually spinning at relatively high revolutions per minute ("RPM"). These days common rotational speeds are 7200 RPM. Rotational speeds in high performance disc drives are as high as 10,000 RPM. Higher rotational speeds are contemplated for the future. These high rotational speeds allows for shorter access times. In other words, as the disc spins faster, the performance of the disc drive is improved since the time necessary to access a particular piece of information on the drive is shortened. Shorter access times are a constant goal of designers and manufacturers of disc drives.
Higher rotational speeds also make the disc drive more susceptible to vibration due to unbalanced rotating parts. As a result, part of the assembly process typically includes the balancing of parts that will rotate within the disc drive. The main rotating part within a disc drive is the disc stack assembly. The disc stack assembly is one or more discs fastened to a spindle hub. Disc spacers and clamps are used to attach the discs to the spindle hub. A spindle motor is used to rotate the hub and disc stack assembly.
It is very important to have a balanced disc pack as an out-of-balance condition results in many problems in a disc drive. One problem associated with an unbalanced disc pack is that it vibrates and causes noise. An unbalanced disc pack also stresses the bearings between the rotating portion of the hub and the spindle shaft. Stressed bearings have a shorter life which may be less than the stated life of the disc drive. An unbalanced disc pack also causes erratic speed variations between the transducing head and the tracks on the disc. These speed variations may result in read/write errors. Planar and axial vibrations of the disc surfaces may also contribute to head crashes.
In addition, planar vibrations or vibrations which travel in the plane of the data surface of the disc make track following of the transducing head difficult. In other words, when the disc stack vibrates in a planar direction, the track to be followed will pass transverse to the tracking direction of the transducing head. The problem is magnified by the fact that the tracks are very closely spaced. In today's disc drives, track densities of 10,000 tracks per inch are common. Six tracks fit on a human hair. This problem will only get worse as time marches on since higher track densities are contemplated for the future. In other words, disc drive performance is boosted by packing the tracks more closely together. The more tracks a disc has, the more information representing data that can be stored. Furthermore, if the vibrational mode has a frequency which is higher than the frequency at which servo sectors are read, the head can cross the track several times between servo sectors.
Prior attempts to solve the unbalanced disc pack problems involved designing and manufacturing discs and hubs to tight tolerances to attain as close a fit between the inner diameter of the discs and the outer diameter of the hub in an attempt to center the rotating mass with respect to the axis of rotation of the disc assembly. This still resulted in out-of-balance conditions because of the inability to attain perfect fits and uniformly balanced discs. To obtain better balance, balance rings were used. By either adding or removing material from the balancing ring as indicated by sophisticated balancing equipment better balance was obtained, but at the expense of further steps following assembly of the disc packs.
U.S. Pat. No. 4,358,803 to Van Der Giessen describes accurately machined inner walls of disc central openings and a centering element which cooperates with at least one of the inner walls to center discs. In U.S. Pat. No. 4,224,648 to Roling, centering was performed using a steel centering ball in the center of a disc pack having a hemispherical surface facing a spindle cup. Centering with respect to the inner walls of a disc does not ensure that the disc is centered with respect to the majority of rotating mass of the discs which lies almost entirely outside the inner wall of the disc. The outer diameter of a disc may not be perfectly concentric with the inner diameter of the disc. Thus, centering with respect to the inner diameter of the discs requires high precision in aligning discs before the fastening of the discs to the hub and neglects some of the major causes of imbalance.
The above methods of centering and fastening discs to form a disc pack also lead to particle generation which degrades performance of disc drives and may even cause a head crash. Mating surfaces, such as the machined inner walls of disc central openings and the centering element in U.S. Pat. No. 4,358,803 and the steel ball and spindle cup in U.S. Pat. No. 4,224,648 are designed to slide while subjected to pressures until mating occurs. This sliding produces undesirable particles.
Another disc pack assembly method for use in a disc drive is taught in U.S. Pat. No. 4,683,505 issued to Schmidt et al. In U.S. Pat. No. 4,683,505 the disc pack has discs which are alternately diametrically offset about a spindle axis of rotation. The discs are positioned as a function of their outer edges such that alternate opposite outer edges line up as though they were the outer edges of centered nominal diameter discs. This results in an increase in the number of axial nodal points for potential imbalance moments and reduces the amplitude of associated vibrations. Disc spacers are also alternately diametrically offset about the spindle axis so that pairs of like components tend to balance each other to minimize potential vibrations. The problem with this solution is that it requires an even number of discs in the disc stack. In addition, the solution does not work in disc stacks having one disc since there is no other disc to counter balance with the only disc. Many of the other solutions require long cycle times which are not favorable to manufacturing.
Thus, there is a need for a method and apparatus for balancing the disc pack having only a single disc. There is also a need for a method that is easy to assemble and easy to manufacture. There is also a need for a method which does not generate particles.